"even the gay girls in the gallery"
Gay girls in the gallery? ('Balcony at the Alhambra' by Gore)
Public DomainGay girls in the gallery? ('Balcony at the Alhambra' by Gore) - Credit: Spencer Gore (1878-1914)

Gay girls is a euphemism for prostitutes (although, as is often the case in Tipping the Velvet, we may read other meanings into the word gay).